An earlier paper (Ref. 1) described the setting of focus in aerial cameras by locating the plane giving maximum modulation at the spatial frequency corresponding to the low contrast resolving power. A modified apparatus has been applied to determine the area weighted average modulation over the whole field (AWAM) as a function of focus, and hence to locate the best compromise focus having regard to field curvature, etc., and to the asymmetry almost invariably present in production lenses. For four types of lens, the best AWAM focus was not significantly changed by the presence of typical amounts of asymmetry. The agreement between the foci determined by the single frequency measurements and those determined by photographic picture quality was within acceptable limits.